Recently, vehicles with a diesel engine that uses a gas oil as a fuel are increased in Europe and in other countries. Therefore, technical developments of fuel injection devices are offered, and advanced performance of common rails is particularly required.
A common rail is a distributing device that retains high-pressured injection fuel, in which a gas oil pumped from a fuel tank by a pump is temporary retained, and thus, the common rail is an important part in a fuel injection device of the diesel engine. The gas oil retained in the common rail is discharged from an orifice (discharging outlet), and is distributed to each injection nozzle provided for respective combustion chamber, via pipes attached to a common rail holder (hereinafter, sometimes simply referred to “holder”). The gas oil transmitted to the injection nozzle is mixed with a combustion air, and then injected to the engine combustion chamber for causing explosive combustion.
In order to improve the combustion efficiency of the gas oil, it is desired to increase the gas oil pressure in the common rail. If the gas oil pressure in the common rail is increased, high power, low fuel consumption, and high torque can be achieved by using a gas oil including a small amount of impurities. Thus, for realizing high pressurized fuel, advanced performance of the common rail is demanded.
Generally, common rails are produced by integrally forging both of a common rail body part and a common rail holder part, and subsequently performing a complicated machining process to define distribution-conduits. In this case, the strength of the common rail can be enhanced by optimizing, for example, chemical components of a steel used for the common rail or production conditions such as heat treatment conditions. Such optimizations make it possible to realize common rails with high reliability that can bear the fuel injection pressure of 160 MPa or more.
However, when the strength of the steel used for the common rail is thus enhanced, the formability and the workability deteriorate, and this results in a high production cost. Therefore, to take the place of conventional forging methods for producing a common rail by means of an integral forming process and a machining process, advanced methods for producing common rails have been demanded. Then, common rail producing techniques in which a common rail is produced by separately preparing a common rail body and a holder, and bonding these separate parts have been suggested.
The Patent Document 1, as shown in FIG. 1, discloses a liquid phase diffusion bonding method that includes a process of arranging an amorphous alloy metal foil 5 between bonding surfaces of a common rail body 1 and a holder 2, and a process of applying a pressure to the holder 2 in the direction indicated by an arrow 60. As shown in FIG. 1, in the common rail body 1, an inner conduit 3 and a branch conduit 4 are provided. Through this inner conduit 3, a fuel (gas oil) pumped from a fuel tank (not shown) by a fuel pump (not shown) is introduced into the common rail body 1 (as indicated by an arrow 11 in FIG. 1). Meanwhile, the inner conduit 3 communicates with a pipe (not shown) which is provided for transmitting the fuel to an injection nozzle (not shown) in an engine combustion chamber via a branch conduit 4 (as indicated by an arrow 12 in FIG. 1).
In FIG. 1, only one branch conduit 4 is given for the sake of convenience, however, a common rail body 1 generally has a plurality of branch conduits 4 that correspond to a plurality of injection nozzles in an engine combustion chamber. Further, a common rail body 1 is generally provided with a plurality of holders 2 that correspond to a plurality of branch conduits 4, in order to connect the plurality of the branch conduit 4 with a plurality of pipes for pumping a fuel to the injection nozzles in the engine combustion chamber. After performing a liquid phase diffusion bonding process for bonding the holder 2 and the common rail body 1, a uniform structure is generated at a bonding portion due to an isothermal solidification. As a result, an excellent bonding quality and an accurate shape are secured and the common rail productivity can be improved.
The Patent Documents 2 and the Patent Document 3 disclose mechanical part manufacturing methods that can be applied for bonding a common rail body and a holder. These methods include a process of arranging an amorphous alloy metal foil between bonding surfaces, a process of performing a heat pressure bonding process (first bonding step), and a process of performing a liquid phase diffusion bonding process (second bonding step). In these methods, a molten pressure-bonded portion formed by the heat pressure bonding process is heated to the melting point of the amorphous alloy metal foil or higher, and then, this temperature is held so as to complete the solidification. These methods make it possible to shorten the time required for the liquid phase diffusion bonding process, and the deformation of the mechanical part can be suppressed.
In addition, the Patent Document 4 discloses a method in which, at the time of performing a first bonding step, a common rail body is fixed to a dedicated jig, and the angle between a welding electrode surface and a bonding surface is adjusted to be 0.03° or less. This method makes it possible to achieve an accurate bonding, since the axis to axis distance between a center axis of a holder and a center axis of a branch conduit in the common rail body, that is, the amount of a bonding displacement becomes small after the first bonding step.